1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic type, in which a unfixed image is transferred onto a recording material at a transfer portion, and then, the unfixed image is fixed onto the recording material in a fixing device.
2. Description of the Related Art
In a conventional image forming apparatus of an electrophotographic type, a unfixed image borne by an intermediate transfer belt 102 has been transferred onto a recording material P by a transfer roller pair 103, to be then fixed onto the recording material P by a fixing roller pair 105, and thereafter, the recording material having the image fixed thereto has been discharged to a discharge tray 107 disposed outside of the apparatus by a discharge roller pair 111, as shown in FIG. 11.
The fixing roller pair 105 has been adapted to heat-fix the unfixed image onto the recording material. Heat temporarily dissipated from fixing rollers during the fixing process has been variously changed according to an amount of adhesion of a developer forming the unfixed image transferred onto the recording material, the number of recording materials passing through the fixing roller pair per unit time a difference in thermal capacity of the recording material, or the like. A temporary change in temperature generated in the fixing roller pair 105 has appeared as a change in outer diameter of the roller. As a consequence, there has possibly occurred a difference between a recording material conveyance speed of the fixing roller pair 105 and a recording medium conveyance speed of the transfer roller pair 103.
In this manner, the difference in recording material conveyance speed between the fixing roller pair 105 and the transfer roller pair 103 has possibly induced degradation of an image quality. In other words, if the recording material conveyance speed of the fixing roller pair 105 is higher than that of the transfer roller pair 103, a tension has been applied to a recording material P between the fixing roller pair 105 and the transfer roller pair 103, so that a blur of an image has occurred when the image is transferred onto the recording material P by the transfer roller pair 103, thereby possibly inducing the degradation of the image quality. In contrast, if the recording material conveyance speed of the fixing roller pair 105 is lower than that of the transfer roller pair 103, the recording material P has formed excessive loop between the fixing roller pair 105 and the transfer roller pair 103. As a result, the recording material P which has formed excessive loop has been forcibly pressed against a conveyance guide 120, to be scraped. Such a scrape on the conveyance guide 120 has generated the blur of the image during the transfer of the image, thereby possibly inducing the degradation of the image quality.
Therefore, the above-described degradation of the image quality has been overcome by preventing any tension from being applied to the recording material P by forming a predetermined loop on recording material P between the transfer roller pair 103 and the fixing roller pair 105 or preventing the recording material P from being excessively flexed, there has been conventionally proposed, as follows:
As shown in FIG. 11, a loop detecting sensor S for detecting a level of a loop of the recording material P has been disposed in the conveyance guide 120 between the fixing roller pair 105 and the transfer roller pair 103, and then, in response to a detection result, the recording material conveyance speed of the fixing roller pair 105 has been switched to a first speed lower than the recording material conveyance speed of the transfer roller pair 103 or a second speed higher than the first speed, thereby keeping the level of the loop within a predetermined range. Such a technique has been disclosed in Japanese Patent Application Laid-open (JP-A) Nos. 5-107966 and 7-234604.
By the technique shown in FIG. 11, when the recording material P is sandwiched and transported between the transfer roller pair 103 and the fixing roller pair 105, an image can be formed while the level of the loop of the recording material P has been kept within the predetermined range by the effect of the difference in speed between both of the roller pairs 103 and 105 in response to the detection result by the loop detecting sensor S.
However, since the level of the loop of the recording material P, formed by the effect of the difference in speed between both of the roller pairs 103 and 105 is constant all the time, the recording material P has bounced up at the rear end thereof due to its own tenacity when the rear end of the recording material P passes through the transfer roller pair 103 in the case of, for example, a high set level of the loop. Such a bounce of the recording material P has caused the blur of the image, thereby possibly inducing the degradation of the image quality.
In order to solve the problem of the bounce of the rear end of the recording material, Japanese Patent Application Laid-open (JP-A) No. 2003-241453 discloses a technique, in which an image is formed while the level of a loop of a recording material is kept within the predetermined range, and further, a conveyance speed is switched to a speed, at which the loop of the recording material is reduced, when the rear end of the recording material reaches a predetermined position in a transfer portion. With this technique, the rear end of the recording material can pass through the transfer portion at a low level of the loop of the recording material, thus to prevent from bounce.
However, the configuration, in which the level of the loop of the recording material is reduced by switching the speed, has raised a problem, as described below.
With this configuration, if a timing of the switch of the conveyance speed for the purpose of the reduction of the level of the loop is shifted or a conveyance condition is varied due to a slippage caused by a type of recording material (i.e., a surface state or a rigidity), the recording material may pass through the transfer portion at various timings. Therefore, in the case where the rear end passes through the transfer portion before the loop becomes satisfactorily small, the bounce of the rear end cannot be securely prevented. In contrast, in the case where the rear end passes through the transfer portion after there is no loop of the recording material, a fixing roller pair has applied a tension to the recording material since the conveyance force of the fixing roller pair is normally greater than that at the transfer portion, thereby possibly inducing a deficient transfer at the transfer portion. As described above, the prior art has experienced the problem that the rear end of the recording material cannot be stably prevented from bouncing up.